An Introduction to the Quaternary Geology and Geomorphology of the Area Around Fort Augustus, Great Glen
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An introduction to the Quaternary geology and geomorphology of the area around Fort Augustus, Great Glen. Jon Merritt and Callum Firth Fort Augustus lies within the Great Glen at the south-western end of Loch Ness (Merritt et al., 2013, fig.17). The settlement straddles the Caledonian Canal, which follows the valley of the River Oich south-westwards towards Loch Oich and, eventually, Fort William. The landforms and deposits in the vicinity of Fort Augustus include drift limits, kame-and-kettle topography and raised lake shorelines. They provide important information for interpreting events that occurred during late-glacial times, in particular, evidence for re-depression of the Earth's crust by the build-up of ice in the western Highlands during the Loch Lomond Stadial (LLS) (Firth, 1986, 1989), and for catastrophic drainage of the former ice-dammed lake in Glen Spean and Glen Roy, some 30 km to the south-west, towards the end of the Stadial (Sissons, 1979a, 1981). Three sites are described here; Borlum (NH 384 084), the ‘north shore’ of Loch Ness (NH 386 105) and Auchteraw (NH 364 082) (Fig. 1). A summary of each site is given below together with some new information obtained from a recent geological survey of the district (BGS, 2012). All modern BGS mapping around Fort Augustus is available digitally or as paper maps at the 1:10,000 scale. The low ground at the southern end of Loch Ness and the surrounding slopes of the Great Glen are mantled by extensive glacial and glaciofluvial deposits (BGS, 2012). Both Charlesworth (1956) and Synge (1977) identified a lateral moraine on the eastern flank of the Great Glen in the vicinity of Fort Augustus, rising from the shore of Loch Ness southwards along the Allt an Dubhair to an altitude of about 100 m OD (Fig. 1). Drift limits also have been identified on the western side of the valley rising south-westwards from Jenkins Park (Fig. 1) (Synge, 1977; Sissons, 1979a, Firth, 1993a; BGS, 2012). Palaeoenvironmental evidence from cores taken from Loch Tarff (NH 425 100) and Loch Oich (NH 330 020) revealed that the typical late-glacial ‘tripartite’ sequence was absent from the latter loch whilst present in Loch Tarff, also in Loch Ness (Pennington et al., 1972). This indicates that the aforementioned moraines formed during the LLS and reveal the northernmost extent of the Loch Lomond Readvance (LLR) within the Great Glen. Raised shorelines and deltas occur sporadically along both sides of Loch Ness, where they have been tilted north-eastwards away from the point of greatest glacio-isostatic uplift centred on Rannoch Moor (Firth, 1989). In general, the gradients of older shorelines are steeper than younger ones owing to the exponential decline of uplift since deglaciation. However, this would not be the case if the rate of uplift was slowed or halted by renewed ice sheet growth. Borlum (NH 384 084) The roadside site at Borlum provides a good vantage point overlooking the southern end of Loch Ness (Fig. 1, exposure A). It lies about 2 km within the LLR limit at Fort Augustus, where Firth (1984) mapped a glacial drainage channel descending through kame-and-kettle topography towards an outwash terrace. The terrace merges into a raised shoreline terrace at 32.4 m OD, capping a steep, erosional bluff adjacent to the road. Originally considered to be of marine origin (Synge, 1977; Smith, 1977), the shoreline is much more likely to be lacustrine (Sissons, 1979a,b; Firth, 1984, 1986). The bluff is fronted by a lower raised shoreline fragment at 22.4 m OD, which shelves towards a raised shingle ridge at 17.9-18.0 m OD, adjacent to the shore of Loch Ness (Sissons, 1979a; Firth, 1986, 1993a). The significance of the elevations of these shorelines is discussed below. Degraded exposures in the roadside bluff (Fig. 1, exposure A) reveal that the high shoreline terrace is underlain by up to 0.5 m of subrounded to well rounded, clast-supported gravel including some cobbles. The gravel wraps around boulders in the underlying deposit, which it overlies unconformably (BGS registered photos P699063-4). The rest of the bluff is composed of up to 6 m of extremely poorly sorted, subangular to rounded boulder gravel, some clasts being over 1 m in diameter. The sequence is interpreted as a lake beach deposit resting on glaciofluvial outwash gravel. Recent mapping has revealed that the high shoreline and related glaciofluvial terrace deposit at Borlum rests on till, which makes up most of the fan-shaped feature hereabout that has been deeply dissected by the River Tarff (BGS, 2012). At two river cliff exposures (Fig. 1, exposures B and C) glaciofluvial gravel underlies the till. Till was also observed in an excavation into a terrace of the River Tarff nearby (NH 3809 0842) (Fig. 1, exposure D) (BGS registered photos P699059-62). The exposure revealed about 0.8 m of very compact, stoney diamicton resting on extremely poorly-sorted boulder gravel several metres thick. These observations suggest that the snout of the LLR glacier may have oscillated, over-riding its outwash. A delta-top terrace at Invermoriston (31.7 m OD), 9 km north-east of Fort Augustus, has been correlated with the highest shoreline at Borlum (Firth, 1984, 1986) (Fig. 2). A landslip scar (NH 4239 1621) cutting the terrace there in 2008 exposed at least 3 m of horizontally bedded, moderately well sorted, sandy gravel resting unconformably on over 2.5 m of fine-grained sand and silt, displaying ‘Type S’ and ‘lee-side preserved’ climbing ripple-drift lamination (cf. Evans and Benn, 2004) with a southwest-directed palaeocurrent (BGS registered photos P699080-3). Although the basal unit is consistent with deltaic deposition (BGS, 2012), the absence of large-scale planar cross-bedded sand and gravel between it and the upper unit suggests that much of any original coarsening-upwards deltaic sequence has been eroded away prior to the deposition of the upper, fluviatile unit. The elevation of the unconformity, which lies approximately 7 m above loch level, does not necessarily indicate former water level as in a classical Gilbert-style deltaic sequence. Inchnacardoch Platform, north shore of Loch Ness (NH 386 105) A prominent bench has been cut into the hillside above the northern shore of Loch Ness in the vicinity of the Inchnacardoch Hotel, north of Fort Augustus (Fig. 1). The bench, named here as the ‘Inchnacardoch Platform’, lies at 29.0-29.5 m OD, is up to 10 m wide, and backed locally by degraded cliffs cut in bedrock; an analogous, less well-developed feature occurs on the other side of the loch (Firth, 1984, 1989). Both features occur outside the likely LLR limit at Fort Augustus. Synge (1977) and Synge and Smith (1980) proposed that the shoreline was a marine feature formed during the decay of the main Late Devensian ice-sheet, when the sea penetrated into Loch Ness from the north-east. They considered that marine terraces occurred throughout the valley of the River Ness, near Inverness, revealing a former link between Loch Ness and the Moray Firth. However, subsequent mapping of the Ness Valley by Firth (1984, 1993b) revealed no evidence there to support a marine incursion into Loch Ness. As the Inchnacardoch Platform is mainly an erosional feature, and there is limited fetch on Loch Ness hereabout, the feature was most probably produced by periglacial shoreline processes during the LLS (Firth, 1986, 1989c: Mathews et al., 1986). If the Inchnacardoch Platform is indeed of LLS age, its elevation and position just outwith the LLR limit at Fort Augustus demonstrates that a rise in loch level occurred from about 29 m to 32 m OD, when the highest shoreline terrace at Borlum was formed, which is within the LLR limit and therefore younger. Together with evidence from the gradients of tilted raised shorelines around Inverness and the Inner Moray Firth (Merritt et al., 1995), this rise demonstrates that the Earth's crust was glacio-isostatically re-depressed regionally during the LLR (Firth 1986, 1989), as suggested by Sutherland (1984). Although shoreline fragments correlating with the Inchnacardoch Platform have been identified at Dores, Lochend and Glen Urquhart (Firth, 1984, 1993b), at the northern end of Loch Ness, some doubt exists about three well-documented raised shingle ridges at Dores. The age, origin and significance of these features has been debated in the literature (Firth, 1993b), whether marine or lacustrine, but they should now be reassessed in the light of the findings of geophysical surveys offshore (Turner et al., 2012) that reveal a cluster of similarly orientated, cross-loch, De Geer moraine ridges lying closely offshore. Much of the Inchnacardoch Platform is now obscured by conifers, but is accessible from the Great Glen cycle track, for which there are several points of access from the A82 trunk road. Of additional interest is a section (Fig. 1, exposure E) (NH 3900 1067), situated behind a private house accessed from the main road. It lies a few metres down-slope from the eastern end of the platform, where it has been dissected by the Allt na Criche, a small stream that flows down the steep northern flank of the Great Glen into Loch Ness (BGS, 2012). In 2008 the exposure revealed over 3.5 m of large-scale planar cross-bedded gravel with beds dipping steeply towards the south-west, some beds revealing normal grading (BGS registered photos P699069-74). The mainly fine, angular to subangular gravel is loose, partly clast-supported and openwork. The deposit has the attributes of the foreset beds of a delta or subaqueous fan (cf.